In general, fluid flows through an area, for example a pipe, in a substantially axial flow pattern if there are no obstructions or other external forces. An object placed in that axial flow pattern creates a disturbance. Vortices then result along both sides of the object as the fluid flows past. Each vortex created sheds from the object as the fluid flow carries it downstream. The generation and shedding of the vortices alternates between the two sides of the object and is continuous with the flow of fluid past the object.
It is possible to sense and measure a low-pressure area associated with the vortex in the fluid flow. It is this characteristic that is the foundation for vortex flowmeters. In a vortex flowmeter the design includes an object, otherwise known as a bluff body, placed in the flow of the fluid. Both sides of the bluff body alternately generate vortices and subsequently shed them. A pressure sensor, such as, for example, a pressure transducer, positioned downstream of the bluff body senses each vortex that is shed from the object. Each time a vortex flows past the pressure transducer, it causes the pressure transducer to generate a pulse having an amplitude proportional to the fluid density and the square of the fluid flow rate. The vortex shedding frequency, i.e., the rate at which vortices are shed, is proportional to the fluid flow rate.
When there is sufficient Reynolds Number and fluid velocity to consistently generate vortices, for example a Reynolds value of 5,000 or higher, simple calculations utilize the vortex shedding frequency to determine the flow rate of a fluid, so long as the rate is constant or has a relatively slow rate of change.
However, if the Reynolds Number of the fluid is generally less than 5,000, the generation of vortices will be either inconsistent, too miniscule for the sensor to measure, or non-existent. A Reynolds Number higher than 5,000 with a low fluid velocity will also create such conditions because fluid velocity that is too low will prevent the sensor from functioning correctly. This is a common problem in vortex metering, which makes it prohibitive to utilize vortex meters for metering situations in which a wide range of flow rates is occurring that includes low-flow rates less than the flow velocities at which consistent, measurable vortices are generated.
This barrier creates several inconveniences to users of vortex flowmeters, which results in the elimination of vortex metering as an option for many applications. Some examples include applications having start-up modes, batching, or intermittent flow rates.
For the foregoing reasons, as well as others not discussed, there is a need for a measuring device an instance of which is a flowmeter having the reliability and features of a vortex-metering device at normal flow conditions, with the added feature of being able to measure flow rate during low-flow to zero-flow conditions.